In the conventional production of integrated circuits a semiconductor substrate is coated with a layer of photoresist and then illuminated through a special photographic plate, the so-called photomask. The photoresist is then developed, as by the removal of its exposed portions and the processing of the underlying semiconductor substrate, e.g. by etching. The photomask contains patterns consisting of a number of lines and various geometrical figures. In most cases, the pattern is divided into several identical configurations--known in the industry as mask elements--each associated with a separate circuit. Mask elements are generally produced by photographic technique, including multiple reduction. The photomasks containing a plurality of mask elements are produced by a high-accuracy step-and-repeat technique using a special camera.
Since this way of producing photomasks is both expensive and time-consuming, it is desirable to exploit the individual photomasks to a maximum extent. Moreover, damaged photomasks have to be withdrawn from production as their use would otherwise result in improperly functioning integrated circuits. To satisfy the extreme resolution requirements of modern integrated circuits, industrial practice uses contact copying of the photomasks both for their reproduction and for their transfer to the photoresist-coated wafer. During contact copying the photomasks touch each other and the photoresist-coated semiconductor, thereby risking damage or contamination. Already during the production of photomasks there may arise defects such as scratches, bubbles, microscopic holes or the like. Consequently, there is a great need for inspecting photomasks to determine whether or not they meet existing requirements.
Besides conventional microscopic testing, methods using coherent illumination are known for inspecting photomasks employed in IC production; see, for example, U.S. Pat. Nos. 3,743,423 and 3,787,117. In one such method, based upon intensity (spatial) filtering, the photomask is illuminated by a spatially coherent light beam which is diffracted when penetrating the photomask. The diffracted light is focused by a lens onto a planar optical spatial filter consisting of discrete opaque areas on a transparent field. The distance between the opaque areas is inversely proportional to the distance between the elements on the photomask. The filter spatially modulates the incident diffraction pattern and suppresses the periodic signal component. The light so modulated is retransformed to yield an image which contains no periodic signal and, consequently, shows an intensity distribution corresponding to the random pattern of defects in the photomask.
The techniques employing intensity (spatial) filtering have the following disadvantages:
(a) only randomly distributed, nonperiodic errors can be detected;
(b) they are effective only if the inspected photomasks contain a large number of elements;
(c) they also detect errors when phase defects are found which do not influence the quality of the manufacturing process.
Another method using coherent illumination utilizes subtractive spatial filtering for inspecting photomasks. A beam of coherent radiation is directed onto the photomask to be modulated and is then passed through a mask containing at least two apertures aligned with the columns of the matrix-like array of photomask elements. Both the aperture widths and the aperture-to-aperture spacing are functions of the element-to-element spacing along the rows of the array. The beam traversing the apertured mask is passed through a lens and then through an optical grating positioned in the focal plane of that lens. The grating has rulings parallel to the major axis of the apertures and is asymmetrical with respect to the optical axis. The grating periodicity is a function of the element-to-element spacing along the rows of the array. Finally, the resulting central and side images of each aperture are displayed on a receiving surface. The centermost side images of each aperture overlap and cancel each other and, hence, the nonperiodic errors of the photomask can be detected.
Accordingly, the disadvantages of this latter method can be summarized as follows:
(a) As the described technique mainly utilizes those parts of the optical elements which lie at a distance from the optical axis, the deviations due to lens aberration, uneven illumination etc. assume added importance and are--erroneously--detected as defects of the photomask.
(b) The optical elements cannot be exploited sufficiently to be able to ensure a high resolution.
(c) The geometric arrangement does not allow comparison of the inspected photomask with a standard photomask because there is not enough space for the displacement of the photomask.
(d) The technique is suitable for the detection of randomly distributed, nonperiodic errors only.
(e) Phase defects in the photomask not detrimental to the production are also detected as errors.
A general disadvantage of the methods using subtractive spatial filtering is that the images to be subtracted arrive in the plane of detection along different optical paths. In practical applications, the inevitable small deviations between the individual optical paths due to, for example, lens aberrations are misinterpreted and detected as errors of the photomask.